Navigational lights utilizing drum lenses are being required to have increasingly greater light outputs. However, there is a maximum practical optical output which can be radiated from a unit of filament surface. In addition, vertical beam spread of light from an ideal drum lens is in direct proportion to the length or height of the lamp filament and high intensity lamp filaments often exceed the minimum length resulting in beam divergencies in an excess of the minimum value required. These problems can be overcome to some extent by utilizing multiple short filaments for increasing the optical output and narrowing the beam of light from the lens.
However, multiple filaments introduced problems of their own. One of the problems of multiple filaments is caused by shadowing. That is, it is desired to maintain an optical output from the lens in a wide horizontal direction, and therefore some of the filaments will be aligned with other filaments causing shadowing or reducing of the light output in the direction of aligned filaments. Furthermore, with a multiplicity of filaments, the filaments will not be positioned on the focal point of the drum lens thereby causing defocusing of the light and consequent undesirable beam spreading.
The present invention is directed to an improved lamp filament configuration for use in drum lenses of a navigational light in which the filaments are positioned to reduce filament shadowing and defocusing, and provide increased light output with a minimum of power.